1. Field of the Invention
The present invention relates to an apparatus which performs control in an internal combustion engine so that the ignition of compressed air/fuel mixture occurs at an optimum crankshaft angle appropriate to the engine operating condition.
2. Description of the Related Art
Generally, in modern automotive engines, not only fuel injection but also ignition timing are controlled electronically. Electronic control of ignition timing permits precise control of the ignition timing compared to the traditional mechanical control type, and achieves improvements in output performance, fuel economy, emission control performance, etc. Computation of such ignition timing is accomplished by obtaining optimum ignition timing by referring to an ignition timing map based on the engine load (the amount of intake air per engine revolution or some other quantity equivalent to such intake air amount) and engine speed.
One such ignition timing control apparatus is disclosed, for example, in Japanese Patent Unexamined Publication No. 1-290967. The apparatus disclosed therein further involves detecting a transient condition based on the rate of change of throttle angle, and controlling the ignition timing during the transient condition by using a transient ignition timing map predetermined based on data obtained by experiment; in this way, the method of ignition timing calculation is changed between the steady-state and transient operating conditions, attempting to suppress deviations from the optimum ignition timing during transient operating conditions.
Normally it would be desirable that the transient ignition timing thus determined based on experimental data and stored in memory in the electronic control unit be optimum for all engines. In reality, however, there are manufacturing variations among component parts even for engines of the same type, and furthermore, each engine is subject to variations with age; as a result, the stored transient ignition timing is not always optimum, and may, in some cases, depart substantially from the optimum ignition timing.
On the other hand, regarding the timing at which the ignition timing is calculated, there are many engines in which the ignition timing is calculated at fixed intervals of time, regardless of the open/close timing of the intake valve (the timing differs from cylinder to cylinder), and ignition in each cylinder is effected based on the most recent calculated value for that cylinder. Normally the desired ignition timing should be calculated based on the engine speed and the amount of air filled into the cylinder by the opening action of the intake valve. However, the most recent calculated value used to calculate the ignition timing is a value based on the engine speed and the intake air amount at the time of the calculation. With this calculation method, while in the steady-state condition the actual ignition timing agrees with the desired ignition timing, in a transient condition the actual ignition timing tends to deviate from the desired ignition timing.
The difference between the actual intake air amount and the intake air amount used for the calculation of ignition timing becomes pronounced during transient operating conditions such as acceleration or deceleration. During acceleration, the intake air amount (more precisely, the intake air mass per engine revolution g/rev.!) increases with each combustion. At the same engine speed, combustion speed increases as the intake air amount increases; as a result, the desired ignition timing shifts toward the retard side. For this reason, if the ignition timing is determined based on the intake air amount measured after the intake valve is closed, ignition occurs later than the desired ignition timing, resulting in reduced torque and poor drivability. On the other hand, during deceleration, contrary to acceleration, ignition occurs earlier than the desired ignition timing, tending to cause knocking. This also worsens drivability.